173. Binary Search Tree Iterator

amortized O(1)

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class BSTIterator {
public:
    BSTIterator(TreeNode *root) {
        pushAllLeft(root);
    }

    /** @return whether we have a next smallest number */
    bool hasNext() {
        return !s.empty();
    }

    /** @return the next smallest number */
    int next() {
        auto node(s.top());
        s.pop();
        pushAllLeft(node->right);
        return node->val;
    }

private:
    void pushAllLeft(TreeNode *root) {
        while (root) {
            s.push(root);
            root = root->left;
        }
    }

    stack<TreeNode *> s;
};

/**
 * Your BSTIterator will be called like this:
 * BSTIterator i = BSTIterator(root);
 * while (i.hasNext()) cout << i.next();
 */

preorder iterator

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class BSTIterator {
public:
    BSTIterator(TreeNode *root) {
        s.push(root);
    }

    /** @return whether we have a next smallest number */
    bool hasNext() {
        return !s.empty();
    }

    /** @return the next smallest number */
    int next() {
        auto p = s.top(); s.pop();
        if (p->right) {
            s.push(p->right);
        }
        if (p->left) {
            s.push(p->left);
        }
        return p->val;
    }

    stack<TreeNode *> s;
};

/**
 * Your BSTIterator will be called like this:
 * BSTIterator i = BSTIterator(root);
 * while (i.hasNext()) cout << i.next();
 */